Well logging system arranged for stable, high-sensitivity reception of propagating electromagnetic waves

ABSTRACT

The present invention is directed to an improved logging apparatus. The logging apparatus includes transmitters, receivers, and a controller member. The controller member is utilized to selectively energize the transmitters to generate an interrogation signal. The receivers are utilized to obtain measurements of the interrogation signal. The controller member can be utilized to substantially simultaneously process the recorded samples--such as by mathematically combining a plurality of measurements in order to obtain a measure of an attribute value which relates to at least one of the wellbore and surrounding formation.

This is a continuation of application Ser. No. 07/820,091 filed on Jan.13, 1992, now U.S. Pat. No. 5,402,068, by Richard A. Meador, James E.Meisner, Ronald A. Hall, Larry W. Thompson, and Edward S. Mumby for"Well Logging System Arranged for Stable, High-Sensitivity Reception ofPropagating Electromagnetic Waves."; which is a continuation under 37C.F.R. 1.60 of U.S. patent application Ser. No. 07/697,524, filed Apr.29, 1991 now U.S. Pat. No. 5,081,419, which is a continuation under 37C.F.R. 1.62 of U.S. patent application Ser. No. 07/595,795, filed Oct.9, 1990, now abandoned, which is a continuation under 37 C.F.R. 1.62 ofU.S. patent application Ser. No. 07/173,239, filed Mar. 24, 1988 nowabandoned.

BACKGROUND OF THE INVENTION

1. Field of The Invention

In general, this invention relates to electrical logging of formationssurrounding a borehole; more particularly, it relates to measuringformation resistivity by processing signals induced in receivingantennas by electromagnetic waves that are caused to propagate throughthe formation.

2. The Prior Art

Data concerning how an electrical parameter such as formationresistivity varies with well depth provide useful clues in exploring foroil and gas bearing beds. Resistivity may be expressed as the ratio ofvoltage gradient (e.g., voltage difference per meter), to currentdensity (e.g., amperes per square meter), and is generally expressed inunits of ohm-meters.

Over the years, many formation-resistivity measuring systems have beendeveloped. The known systems may be classified in numerous ways.According to one basis for classification, a system can be categorizedas either a wireline system or as a measure while drilling (MWD system).Such systems can also be categorized with respect to frequency ofoperation of electrical signals used in effecting measurement offormation parameters.

Measuring while drilling has significant, long-recognized advantages.For the measurement of formation resistivity, a MWD system isparticularly advantageous in reducing or eliminating the adverse effectthat can be caused if drilling fluid (referred to as "mud") has hadsufficient time to invade the formation before the resistivitymeasurement is made. A MWD system is advantageously incorporated in alogging collar section or sub that is positioned in the drill stringvery close to the drill bit. Thus, relatively little time elapses fromwhen the drill bit cuts through a region and when the sub is broughtinto position to effect a measurement of a parameter of the formation inthat region. Another advantage of a MWD system is that data for anyformation parameter it measures that can be communicated to the surfaceduring drilling operations via mud pulse telemetry techniques.

Such advantages, however, can be gained only if the MWD system iscapable of withstanding the extremely adverse environmental conditionsprevailing down hole while drilling. The adverse environmentalconditions involve high temperature and shock. Further, during thedrilling operation, mud is circulated under high pressure to flow downthrough a passageway within the drill string to, and out of the drillbit, and then to flow back up in the annular space between the drillstring and the wall of the borehole, carrying cuttings to the surface.Various elements of a MWD system must be contained in an electronicshousing, which is a sealed pressure vessel or barrel, that is anchoredin place within the MWD sub and that protects components such aselectronic circuits from exposure to the high pressure mud. Further, anyelement of the MWD system that is exposed to the upwardly flowing mud,and the whipping action of the drill string against the inner wall ofthe borehole, must be extremely abrasion resistant.

Another difficulty associated with a MWD system is that the electricalpower for the MWD system is generated within a downhole drilling segmentby a turbine-driven generator, the turbine being driven by thedownflowing mud. Because the electrical power is generated inside thedrill string segment while high pressure drilling mud is circulatingdown inside the drill string segment and up about its outside,complexities arise in distributing electrical power and electricalsignals to various components of the MWD system.

In contrast to a MWD system, far fewer problems need to be addressed ina well logging system, commonly called a wireline system, that is usedwhile drilling operations are suspended. Because the mud is stationarywhile drilling operations are suspended, various elements of a wirelinesystem are not subjected to the adverse conditions discussed above. Oneminor exception is that downhole temperature is somewhat higher whilethe mud is stationary than while the mud is circulating and to someextent providing cooling. The environmental conditions of use of awireline system, in addition to being generally more benign, enablesubstantially more control over distribution of electrical power. In awireline system, a generator is located at the surface, and the electricpower it generates is easily supplied to downhole electronics.

As stated above, another way of classifying a system is on the basis ofthe frequency of operation of an electrical signal used to effect themeasurement. With respect to the lowest end of the frequency spectrum,there are electrode systems that involve a frequency of operation in theneighborhood of about 1 KHz. Each of these low frequency systems reliedupon conduction of current through the mud as part of a current flowpath that also includes electrodes and the surrounding formation.Because the mud forms part of this current flow path, an electricallyconductive mud such as a water base mud is necessary for properoperation of any of these systems. In many drilling operations, it isundesirable to use such a water base mud, and instead it is desirable touse an oil base mud that has very high resistivity.

Examples of wireline electrode systems are described in two papersauthored by Hubert Guyod. One of these, titled "The Shielded ElectrodeMethod," appears in the December 1951 issue of World Oil, at pages111-116. The other, titled "Factors Affecting the Responses ofLaterolog-Type Logging Systems (LL3 and LL7)," appears in the February1964 issue of the Journal of Petroleum Technology, at pages 211-219.Examples of MWD electrode systems are described in U.S. Pat. No.4,570,123 to Grosso. An improved MWD electrode system has been developedby the assignee of this invention and is disclosed and claimed in a U.S.patent application Ser. No. 025,937, filed Mar. 16, 1987, entitled "WELLLOGGING SYSTEM EMPLOYING FOCUSED CURRENT IN MEASURING RESISTIVITY WHILEDRILLING"; the inventors being J. Meisner, et al.

With respect to the next frequency range in the spectrum, there areinduction systems that involve a frequency of operation in theneighborhood of 20 KHz. An induction logging system generates a magneticfield in the formation to produce a secondary current flow in theformation. The secondary current flow sets up a second magnetic fieldwhich induces current in receiving coils in proportion to the secondarycurrent flow in the formation and thus the induced current is directlyproportional to the resistivity of the surrounding formation.

An induction logging system uses large diameter coils to obtain thenecessary coupling. To apply induction logging techniques in a MWDsystem, inductive logging coils must be mounted in or about a drillcollar in a drill string and that portion of the collar must benon-conductive. It is difficult to build a non-conductive collar thathas the structural integrity and strength necessary for use in a drillstring.

With respect to a much higher range of frequencies in the spectrum,there are electromagnetic wave propagation (EWP) systems that involve afrequency of operation in the range of about 500 KHz to about 4 MHz. AnEWP system is disclosed in U.S. Pat. No. 3,551,797 to Gouilloud et al.The EWP system disclosed in the Gouilloud patent is a wireline systemhaving a transmitter and receivers for measuring formation parameters,and utilizing phase comparison and amplitude. U.S. Pat. Nos. 4,107,597and 4,185,238 also show EWP wireline systems.

Each of the foregoing wireline systems involves a non-conductive sonde.Because the sonde is non-conductive, it does not significantly reducethe signal strength of the electromagnetic wave signal used to effectthe measurement of formation resistivity. Circumstances are different inthe case of a MWD system in which electromagnetic wave propagation isused to accomplish the measurement of formation resistivity. A MWDsystem necessarily involves a metal drill collar, and because the metaldrill collar is highly conductive it can significantly reduce the signalstrength of the receiver signals derived from the electromagnetic wavesignal used to effect the measurement. The problem of dealing with verylow signal strengths can be aggravated by the presence of electricalnoise.

A published U.K. Patent Application No. GB 2,146,126A is directed aspecific electrostatic shielding arrangement for reducing the adverseeffect of noise in a MWD system that uses electromagnetic wavepropagation for measuring formation resistvity. A paper relating to thisMWD system was presented at a SPE conference in San Francisco, Calif.,Oct. 5-8, 1983; this paper is titled "The Electromagnetic WaveResistivity MWD Tool"; its authors are P. F. Rodney et al. According tothe published U.K. patent application, the preferred embodiment of thespecific electrostatic shielding system is provided in a MWD EWP systemthat includes a drill collar that has an outside diameter of 17.8 cm,and has two axially-spaced apart cylindrical annular recesses each ofwhich has a diameter of 14.6 cm. Each of the recesses is filled in withnitrile rubber. A transmitting antenna is embedded in the nitrile rubberthat fills in one of the recesses, and two axially-spaced apart receiverantennas are embedded in the nitrile rubber that fills in the otherrecess. Each antenna has an inner diameter of 15.75 cm. Thus, there is aminimum spacing of about 0.55 cm. between an antenna and the drillcollar. The axial spacing between the transmitting antenna and thenearer of the two receiving antennas is 24 inches, and the more remoteantenna coil is an additional 6 inches further away. Each antenna isalmost completely surrounded by a corresponding one of threeelectrostatic shields that are electrically isolated from the metaldrill collar and electrically connected by conductors in coax cablesextending to signal processing circuitry to ground (0 volts).

The performance capabilities of a well logging system can be judged interms of various factors. One factor is ease of use; others are depth ofinvestigation, dynamic range, resolution with respect to delineatingnarrow beds, and the extent to which measurements are independent ofextraneous matters such as borehole effects.

As stated above, some well logging systems are not adapted for use incircumstances in which an oil base mud is being used. As to prior artEWP systems that are adapted for use in such circumstances, there arevarious problems. One of these problems is that the response of eachantenna has to be extremely stable with respect to temperaturevariations. The known design principles applicable to such a prior artsystem are to effect a tradeoff of antenna sensitivity in favor ofantenna stability. This in turn results in other sacrifices. In thisregard, it is desirable to provide substantial axial spacings betweenthe transmitting antenna and the receiving antennas. However, therelatively low sensitivity of the antennas used in prior art systemsimposes a limit on how far apart the antennas can be spaced.

Such shortcomings are widely recognized, and there has been alongstanding need for an improved EWP system to overcome theseshortcomings.

SUMMARY OF THE INVENTION

This invention provides an improved system and method for measuring aformation parameter.

Defined broadly in apparatus terms, the invention resides in apparatusfor use in a borehole to measure an electrical parameter of theformation surrounding the borehole. The apparatus comprises housingmeans, and first and second transmitting antenna means, each arrangedaround the housing means and spaced from each other. The apparatusfurther comprises transmitting circuit means for alternately supplyingan oscillating drive signal to the first and second transmitting antennameans. The oscillating electrical drive signal has a frequencysufficiently high such that one and then the other of the transmittingantenna means radiates an electromagnetic wave signal that propagatesthrough the formation. The apparatus further comprises first and secondreceiving antenna means for producing first and second pick-up signals.Each receiving antenna means is arranged around the housing means at aspaced position between the first and second transmitting antenna means.Each receiving antenna means includes antenna circuit means in which aninduced alternating current is produced while the electromagnetic wavesignal propagates through the formation, tuning means for reducingimpedance of the antenna circuit means in a frequency region embracingthe frequency of the oscillating drive signal, and means responsive tothe induced alternating current for producing a respective one of thepick-up signals. The apparatus further comprises sampled-data signalprocessing means responsive to the pick-up signals for producing asignal that is a function of the formation parameter.

Defined broadly in method terms, the invention resides in a method formeasuring an electrical parameter of the formation surrounding aborehole. The method comprises the step of positioning in the boreholefirst and second receiving antenna that are each tuned to increase thesensitivity of the antenna at a predetermined frequency, and the step ofgenerating an oscillating electrical drive signal at the predeterminedfrequency. The method further comprises the step of alternately couplingthe electrical drive signal to one and then another of a pair oftransmitters to cause an electromagnetic wave signal to propagatethrough the formation in first one direction and then another so thatthe propagating electromagnetic wave signal induces the first and secondreceiving antennae to develop a first pair and then a second pair ofpick-up signals. The method further comprises processing steps includingprocessing the first pair of pick-up signals to produce a first sampledsignal, and recording the first sampled signal; processing the secondpair of pick-up signals to produce a second sampled signal; andprocessing the recorded first sampled signal and the second sampledsignal to produce a signal that is a function of the formationparameter.

A preferred embodiment of the invention resides in ameasure-while-drilling system for measuring resistivity of a formationsurrounding a borehole. In the preferred embodiment, the housing meansis defined by a tubular steel drill collar section. Each of the firstand second transmitting antenna means is arranged around the drillcollar section and they are axially spaced from each other. Thetransmitting circuit means includes circuit means contained within thedrill collar section and coupled to the first and second transmittingantenna means to supply, on an alternating basis, the oscillating drivesignal such that one and then the other of the transmitting antennameans radiates an electromagnetic wave signal that propagates throughthe formation. Each receiving antenna means is arranged around the drillcollar section at an axially spaced position between the first andsecond transmitting antenna means and each includes antenna circuitmeans in which an induced alternating current is produced while theelectromagnetic wave signal propagates through the formation. Eachreceiving antenna means includes an antenna conductor and a capacitorinterconnected to define a single-turn, tuned antenna. The capacitorcooperates with the antenna conductor to provide an extremely lowimpedance in the frequency region embracing the frequency of theoscillating drive signal. The means in each antenna receiving means forproducing a pick-up signal includes a transformer having a core coupledto the antenna conductor. The sampled-data signal processing meansincludes means for producing a phase-representing signal representingthe phase difference between the pick-up signals produced by the firstand second receiving antenna means. Further, the sampled-data signalprocessing means includes sequentially operating processing means forrecording at least one phase-representing signal and thereafterprocessing such stored phase-representing signal together with asubsequently produced phase-representing signal to produce the signalthat is a function of the formation resistivity.

The invention has significant advantages in that it provides for stable,high-sensitivity reception of propagating electromagnetic waves. Tuningeach receiving antenna means enables an increase in antenna sensitivityby a factor in excess of ten. An important benefit of thehigh-sensitivity of the receiving antenna means is that long spacingscan be used between each tuning antenna means and the receiving antennameans closest to it. Such long spacings are advantageous in reducingadverse effects on measurement accuracy that are generally caused byborehole and invaded zone variations. Further, tuning each receivingantenna means significantly reduces its impedance, which in turn reducesadverse electrostatic effects. As a result, electrostatic shieldingarrangements are optional, and if provided, need not involve complexassemblies for insulating electrostatic shielding elements from themetal drill collar in a MWD system.

The foregoing and other features and advantages of the invention aredescribed in detail below and recited in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an overall system for simultaneously drilling and logging awell, in which a logging collar includes a formation-resistivitymeasuring system according to this invention;

FIG. 2 comprises FIGS. 2A and 2B which are schematic, side elevationviews, partly in cross-section, of a top and bottom portion,respectively, of a drill collar section or sub in accord with thepreferred embodiment of the invention;

FIG. 3 is an enlarged schematic, cross-sectional side elevation view ofa portion of the sub illustrated in FIG. 2;

FIG. 4 is a schematic cross-sectional view taken along line 4-4' of FIG.2B, illustrating a receiver conductor encapsulated in insulatingmaterial surrounding a portion of the sub;

FIG. 5 is a schematic cross-sectional view taken along line 5-5' of FIG.4, illustrating a junction box containing a tuning capacitor and othercomponents used to implement a receiving antenna means in the preferredembodiment;

FIG. 6 is a simplified functional block diagram illustrating the overallorganization of signal transmitting, receiving, and processing circuitryincorporated in the preferred embodiment of the invention;

FIG. 7 is a schematic and block diagram of a portion of controlcircuitry incorporated in the preferred embodiment to provide switchingcontrol such that, on an alternating basis, one and then anothertransmitting antenna means transmits an electromagnetic wave signal topropagate into and through the formation surrounding the borehole;

FIG. 8 is a block diagram of the circuitry that is replicated for thefirst and second transmitting antenna means as illustrated in FIG. 6;

FIG. 9 is a block diagram of a phase-locked local oscillator signalgenerator in the receiving circuitry illustrated in FIG. 6.

FIG. 10 is a block and schematic diagram of signal processing circuitrythat is replicated for the first and second receiving antenna means inthe preferred embodiments;

FIG. 11 is a block and schematic diagram of signal processing circuitrythat responds to the replicated circuitry of FIG. 10 to producemulti-scale signals, each representing a phase difference; and

FIG. 12 is a plot of experimental data showing the relationship betweenphase difference and resistivity for a representative embodiment of theinvention.

DETAILED DESCRIPTION

By way of introduction, certain principles underlying important featuresof this invention are indicated by the following generally applicableanalysis.

First, consider four transmitter-to-receiver signals:

(Transmitter 1 X 1! to Receiver 1 R1!): A₁₁ e^(i).o slashed..sbsp.11

(Transmitter 1 X 1! to Receiver 2 R2!): A₁₂ e^(i).o slashed..sbsp.12

(Transmitter 2 X 2! to Receiver 1 R1!): A₂₁ e^(i).o slashed..sbsp.21

(Transmitter 2 X 2! to Receiver 2 R2!): A₂₂ e^(i).o slashed..sbsp.22

The measured amplitudes are made up of:

    A.sub.mn =X.sub.m R.sub.n a.sub.tmn                        (Eq. 1.1)

where

X_(m) =transmitter output variation

R_(n) =receiver sensitivity variation

a_(tmn) =true amplitude (transmitter M to receiver N);

and the measured phases are made up of:

    .o slashed..sub.mn =.o slashed..sub.Xm +.o slashed..sub.Rn +.o slashed..sub.tmn                                          (Eq. 1.2)

where

.o slashed.Xm=transmitter phase (output) variation

.o slashed._(Rn) =receiver phase variation

.o slashed._(tmn) =true phase (transmitter M to receiver N)

The foregoing general equations correspond to the following morespecific equations:

    A.sub.11 =X.sub.1 R.sub.1 a.sub.t11

    A.sub.12 =X.sub.1 R.sub.2 a.sub.t12

    A.sub.21 =X.sub.2 R.sub.1 a.sub.t21

    A.sub.22 =X.sub.2 R.sub.2 a.sub.t22

    .o slashed..sub.11 =.o slashed..sub.X1 +.o slashed..sub.R1 +.o slashed..sub.t11

    .o slashed..sub.12 =.o slashed..sub.X1 +.o slashed..sub.R2 +.o slashed..sub.t12

    .o slashed..sub.21 =.o slashed..sub.X2 +.o slashed..sub.R1 +.o slashed..sub.t21

    .o slashed..sub.22 =.o slashed..sub.X2 +.o slashed..sub.R2 +.o slashed..sub.t22

Taking ratios of the various transmitter-to-receiver signals producesthe following:

For Transmitter 1: ##EQU1## and for Transmitter 2: ##EQU2##

Multiplying these and taking the square root gives: ##EQU3##

Straightforward algebraic manipulation of Eqs. 1.1 through 1.3 yields:##EQU4## because all the system variables drop out of the measurement.

Therefore, by using two transmitters and two receivers, systematicvariables can be removed from both the attenuation (amplitude) and fromthe phase velocity (phase difference) terms.

Within the context of the preferred embodiment of this invention, inwhich a sampled-data processing means produces a signal as a function offormation resistivity based on phase-representing signals, the followinganalysis demonstrates certain matters relevant to the stability feature.

Consider two consecutive samples, i.e., Sample A and Sample B.

During Sample A, a first transmitting coil is energized to cause a waveto propagate through the formation in a direction such that the wavepasses a first receiving coil (R1), and later passes a second receivingcoil (R2), and induces each receiver coil to produce a signal.

During Sample B, a second transmitting coil is energized to cause a waveto propagate through the formation in a direction such that the wavepasses the second receiving coil (R2), and later passes the firstreceiving coil (R1), and induces each receiver coil to produce a signal.

Let .o slashed.MR2A represent the measured phase of the signal producedby receiver coil R2 during Sample A; let .o slashed.MR1A represent themeasured phase of the signal produced by receiver coil R1 during SampleA; let .o slashed.MR1B represent the measured phase of the signalproduced by receiver coil R1 during Sample B; and let .o slashed.MR2Brepresent the measured phase of the signal produced by receiver coil R2during Sample B.

The .o slashed.MR2A signal depends on the phase of the wave at thelocation of R2, and in general, has an error component attributable tovarious phase shifts including those introduced by the tuned receivercoil, cabling from the receiver coil to the receiver, and the receiveritself. Let .o slashed.TR2A represent the true phase of the wave at thelocation of R2 during sample A, and let .o slashed.R2E represent theerror component so introduced.

    .o slashed.MR2A=.o slashed.TR2A+.o slashed.R2E             Eq. 2.1

Similarly, the .o slashed.MR1A signal depends on the phase of the waveat the location of R1, and in general, has its own error component. Let.o slashed.TR1A represent the true phase of the wave at the location ofR1 during Sample A, and let .o slashed.R1E represent the error componentso introduced.

    .o slashed.MR1A=.o slashed.TR1A+.o slashed.R1E             Eq. 2.2

During Sample A, the .o slashed.MR1A signal and the .o slashed.MR2A aresimultaneously processed to produce a DeltaA signal that represents thedifference in phase between these two signals (i.e., .o slashed.MR1A-.oslashed.MR2A).

    DeltaA=(.o slashed.TR2A-.o slashed.TR1A)+(.o slashed.R2E-.o slashed.R1E)Eq. 2.3

The component of the DeltaA signal representing the true phasedifference (.o slashed.TR2A-.o slashed.TR1A) is a function of theresistivity of the formation in the region between the two receivercoils. Let f(rho) represent this component.

    DeltaA=f(rho)+(.o slashed.R2E-.o slashed.R1E)              Eq. 2.4

Similarly, during Sample B, the .o slashed.MR2B signal and the .oslashed.MR1B are simultaneously processed to produce a DeltaB signalthat represents the difference in phase between these two signals (i.e.,.o slashed.MR2B-.o slashed.MR1B).

    .o slashed.MR1B=.o slashed.TR1B+.o slashed.R1E             Eq. 2.5

    .o slashed.MR2B=.o slashed.TR2B+.o slashed.R2E             Eq. 2.6

    DeltaB=(.o slashed.TR1B-.o slashed.TR2B)+(.o slashed.R1E-.o slashed.R2E)Eq. 2.7

The component of the DeltaB signal representing the true phasedifference (.o slashed.TR1B-.o slashed.TR2B) is a function of theresistivity of the formation in the region between the two receivercoils; i.e., it equals f(rho).

    DeltaB=f(rho)+(.o slashed.R1E-.o slashed.R2E)              Eq. 2.8

The DeltaA signal is recorded so that it can be retrieved and processedwith the DeltaB signal.

By adding Equations 2.7 and 2.8, it follows that:

    DeltaA+DeltaB=2*f(rho)+.o slashed.R2E-.o slashed.R1E-.o slashed.R2E+.o slashed.R1E

and

    f(rho)=1/2*(DeltaA+DeltaB)                                 Eq. 2.9

In other words, a computed signal representing the sum of theconsecutive samples is a function of formation resistivity, and errorcomponents such as .o slashed.R1E and .o slashed.R2E do not introduceerrors into this computed signal.

With reference to FIG. 1, there will now be described an overallsimultaneous drilling and logging system that incorporates anelectromagnetic wave propagation (EWP) resistivity measurement systemaccording to this invention.

A well 1 is being drilled into the earth under control of surfaceequipment including a rotary drilling rig 3. In accord with aconventional arrangement, rig 3 includes a derrick 5, derrick floor 7,draw works 9, hook 11, swivel 13, kelly joint 15, rotary table 17, anddrill string 19 that includes drill pipe 21 secured to the lower end ofkelly joint 15 and to the upper end of a section of drill collarsincluding an upper drill collar 23, an intermediate drill collar or sub(not separately shown), and a lower drill collar or sub 25 immediatelybelow the intermediate sub. A drill bit 26 is carried by the lower endof sub 25. To provide dual systems for measuring formation resistivity,the intermediate sub can be made in accord with the teachings of theabove-mentioned U.S. patent application Ser. No. 025,937, filed Mar. 16,1987, and entitled "WELL LOGGING SYSTEM EMPLOYING FOCUSED CURRENT INMEASURING RESISTIVITY WHILE DRILLING"; the inventors being J. Meisner etal., the disclosure of which is incorporated by reference herein.

Drilling fluid (or mud, as it is commonly called) is circulated from amud pit 27 through a mud pump 29, past a desurger 31, through a mudsupply line 33, and into swivel 13. The drilling mud flows down throughthe kelly joint and on axial tubular conduit in the drill string, andthrough jets (not shown) in the lower face of the drill bit. Thedrilling mud flows back up through the annular space between the outersurface of the drill string and the inner surface of the borehole to becirculated to the surface where it is returned to the mud pit through amud return line 35. A shaker screen (not shown) separates formationcuttings from the drilling mud before it returns to the mud pit.

The overall system of FIG. 1 uses mud pulse telemetry techniques tocommunicate data from downhole to the surface while the drillingoperation takes place. To receive data at the surface, there is atransducer 37 in mud supply line 33. This transducer generateselectrical signals in response to drilling mud pressure variations, andthese electrical signals are transmitted by a surface conductor 39 to asurface electronic processing system 41.

As explained in U.S. Pat. No. 4,216,536 to More (More '536 patent), mudpulse telemetry techniques provide for communicating data to the surfaceabout numerous downhole conditions sensed by well logging transducers ormeasurement systems that ordinarily are located on and within the drillcollar nearest the drill bit. Sub 25 is preferably nearest the drillbit, as shown in FIG. 1. The mud pulses that define the data propagatedto the surface are produced by equipment within the intermediate sub.Such equipment suitably comprises a pressure pulse generator operatingunder control of electronics contained within an instrument housing toallow drilling mud to vent through an orifice extending through thelogging collar wall. Each time the pressure pulse generator causes suchventing, a negative pressure pulse is transmitted to be received bysurface transducer 37. An alternative conventional arrangement generatesand transmits positive pressure pulses.

The circulating drilling mud provides a source of energy for aturbine-driven generator sub-assembly located in the intermediate sub,and the turbine-driven generator sub-assembly generates electrical powerfor the pressure pulse generator and for various circuits includingcircuits forming part of the preferred embodiment of this invention.

A measurement system embodying this invention includes electronicscontained in electronics housings contained within the axial tubularconduit of sub 25, and contains elements arranged in recesses ornecked-down portions of the tubular steel housing of sub 25. Some ofthese elements on sub 25 are indicated in FIG. 1, and include fourantenna insulating sleeves 43, 45, 47, and 49, each of which surrounds alongitudinally-extending, necked-down portion of sub 25.

As shown schematically in the cross-sectional views of FIGS. 2A and 2B,sub 25 comprises a tubular steel housing 51 which has an axial bore 53to provide a passage for downflowing drilling mud.

A conductor 55 is part of a first transmitting antenna assembly which isencapsulated in antenna-insulating sleeve 43 that surrounds acircumferential recess 57 in tubular steel housing 51. A conductor 59 ispart of a second transmitting antenna assembly which is encapsulated inantenna-insulating sleeve 49 that surrounds a circumferential recess 61in tubular steel housing 51. A conductor 63 is part of a first receivingantenna assembly which is encapsulated in antenna-insulating sleeve 45that surrounds a circumferential recess 65 in tubular steel housing 51.A conductor 67 is part of a second receiving antenna assembly which isencapsulated in antenna-insulating sleeve 47 that surrounds acircumferential recess 69 in tubular steel housing 51.

The ring-shaped portion of the collar that separates recess 65 fromrecess 69 provides for de-coupling between The first and the secondreceiving antenna assemblies. An alternative arrangement involves asingle recess for both the first and the second receiving antennaassemblies. In the alternative arrangement, the receiving antennaassemblies can be spaced closer together.

Suitably, each antenna-insulating sleeve is made of multiple layersincluding an outer layer of nitrile rubber, a material which issufficiently durable to protect the encapsulated antenna coil from weardespite the adverse conditions involved in a drilling operation, andprovides adequate electrical insulation despite the hydrostaticpressures involved in the drilling operation. A suitable way to makeeach sleeve involves several steps including wrapping around the recessa durable fiberglass of the type that is used in replaceable insulatingsleeves for MWD subs. Then, portions of the fiberglass wrapping are cutaway to provide circumferential and longitudinal grooves for conductorsof the antenna assembly and to provide a recess for a junction box.After insertion of the components of the antenna assembly, the nitrilerubber is applied.

Suitably, the axial spacing from conductor 55 of the first transmittingantenna assembly to conductor 63 of the first receiving antenna assemblyis 28 inches, from conductor 63 to conductor 67 of the second receivingantenna assembly is 10 inches, and from conductor 67 to conductor 59 ofthe second transmitting antenna assembly is 28 inches. In theabove-mentioned alternative arrangement in which both receiving antennaassemblies are in a single recess, the spacings suitably are 30 inches,6 inches, and 30 inches.

Within axial bore 53, there are arranged three pressure-sealedelectronics housings 71, 73, and 75, together with supporting blocks 77and 79. Each supporting block has opposite side surfaces that each has aconvex shape to abut portions of the interior cylindrical surface of sub25. Each remaining side surface of each supporting block is generallyflat, and is spaced from the interior cylindrical surface of sub 25 toprovide a passageway for downflowing drilling mud. Supporting blocks 77and 79 are fixed in place by sealed anchor bolts 81, 83, 85, and 87.

As shown in the enlarged view of FIG. 3, supporting block 79 has anaxial bore 89 that serves as section of a conduit assembly forconductors that extend from circuitry in electronics housings 73 and 75.Axial bore 89 communicates with openings 91 (FIG. 2B) in apressure-sealed, generally radially-extending port connector assembly 93that serves as a section of the conduit assembly for conductors thatinterconnect circuitry within electronics housing 75 and the secondtransmitting antenna assembly. Other sections of this conduit assemblyare a port tube 95, a tubing length adjuster 97, and tubing 99 thatterminates in a junction box 101.

Axial bore 89 also communicates with openings 103 (FIG. 2B) in anotherpressure-sealed, generally radially-extending port connector assembly105 that serves as a section of the conduit assembly for conductors thatinterconnect circuitry in electronics housing 73 and the first andsecond receiving antenna assemblies. Other sections of this conduitassembly are a port tube 107, a tubing length adjuster 109, and tubing111 that terminates in a junction box 113.

With reference to FIGS. 4 and 5, there will now be described theconstruction of the second receiving antenna assembly. Conductor 67 anda tuning capacitor 115 are interconnected to define a single-turn, tunedreceiving antenna. Suitably the capacitance value of capacitor 115 is0.012 microfarads. In combination with a conductor forming a one-turnloop of 61/2 inch diameter, tuning capacitor 115 makes one receivingantenna highly sensitive in the frequency region embracing 2 Mhz. Thisis so because the capacitive reactance is equal (but opposite in phasefrom) the inductive reactance and therefore the loop impedance isminimum (and essentially resistive). In operation, an alternatingcurrent is induced in the loop circuit defined by conductor 67 andcapacitor 115 while an electromagnetic wave propagates through theformation. The magnitude of this alternating current depends on, amongother things, the impedance of the loop circuit. At 2 Mhz, the impedanceof the loop circuit is about 0.5 ohms. Within junction box 113,conductor 67 extends through the aperture of a ferrite ring 117 thatdefines the core of a high efficiency transformer. Conductor 67 definesthe primary of the transformer. A toroidal winding 119 defines thesecondary of the transformer and provides a receiver pick-up signal thatis coupled to receiver circuitry via a coax cable 121. The firstreceiving antenna assembly has the same construction as the secondreceiving antenna assembly, and a coax cable 123 extends from it throughjunction box 113 as shown in FIG. 5 to couple the pick-up signal fromthe first receiving antenna assembly to receiver circuitry.

With reference to the simplified functional block diagram of FIG. 6,there will now be described general features of the overall organizationof signal transmitting, signal receiving, and signal processingcircuitry incorporated in the preferred embodiment.

As stated above, a MWD system embodying this invention preferablyincludes a turbine-driven generator that converts mechanical powersupplied by downflowing drilling mud to electrical power. In aconventional and well known manner, a DC regulator responds to thegenerator and supplies DC power which, although regulated to someextent, is subject to a fairly substantial variation in voltage. This DCpower is applied to a DC-to-AC converter 161. A quartz crystal 163 thatresonates at 20 Khz Is coupled to circuitry 161 so that the ACpower-supply voltage it supplies is at or very close to 20 Khz, and isin the form of a square wave. As explained further below, this ACpower-supply voltage defines a frequency reference, and is accordinglysometimes referred to as a frequency reference (FR) signal.

In a conventional and well known manner, the FR signal is coupled viatransformers and connectors so that 20 Khz AC power-supply voltage isavailable to provide power to circuitry located in each electronicshousing.

The circuitry located in electronics housing 71 (FIG. 2A) includes drivecircuitry 165 that is coupled to supply an oscillating drive signal tothe first transmitting antenna means 167. The circuitry located inelectronics housing 73 (FIG. 2B) includes receiving circuitry 169, powersupply circuitry 171, and data processing and timing and controlcircuitry 173. Receiving circuitry 169 is coupled to the first andsecond receiving antenna means 175 and 177. The circuitry located inelectronics housing 75 (FIG. 2B) includes drive circuitry 179 that iscoupled to supply an oscillating drive signal to the second transmittingantenna means 181.

Circuitry 173 includes a microprocessor and associated circuitry, whichtogether perform numerous functions, including a basic timing functionfor sequencing the alternating operation of the first and secondtransmitting antenna means.

The microprocessor provides a two-bit wide transmitter select signalcomprising an A bit and a B bit that are coded as follows: if the A bitand the B bit have the same binary value (whether both "1" or both "0")then this represents a command to turn both transmitters off; if the Abit is "1" and the B bit is "0" then this represents a command to turntransmitter 1 on and turn transmitter 2 off; and, if the A bit is "0"and the B bit is "1" then this represents a command to turn transmitter1 off and turn transmitter 2 on. In the preferred embodiment, a "1"binary value is represented by +5 volts, whereas a "0" binary value isrepresented by 0 volts (ground).

In the preferred embodiment, the digitally-coded transmitter selectsignal is converted to an analog signal by circuitry within electronicshousing 73 and coupled to circuitry and electronics housings 71 and 75in a highly advantageous way. In this regard, reference is made to FIG.7. A weighting circuit generally indicated at 185 in FIG. 7 comprises aresistor 187, a resistor 189, a pair of protection diodes 191 and 193, aresistor 195, and a capacitor 197, all of which are connected to acommon node point 199 that is connected to the center tap of a primarywinding of a transformer 201. The resistance values of resistors 185,187, and 195 are selected such that the potential at node point 199constitutes an analog control signal.

Suitable resistance values are 1K ohm for resistor 187, 2K ohm forresistor 189, and 10K ohm for resistor 195. If each of the A bit and theB bit signals is +5 volts, then the potential at node point 199 is +4.69volts. If each of the A bit and the B bit signals is 0 volts, then thepotential at node point 199 is 0 volts. If the A bit signal is +5 voltsand the B bit signal is 0 volts, then the potential at node point 199 isat or approximately +3.13 volts. If the A bit signal is 0 volts and theB bit signal is +5 volts, then the potential at node point 199 is at orapproximately +1.56 volts. The opposite ends of the primary winding oftransformer 201 are connected to conductors 205 and 207 that define anAC power bus within electronics housing 73. A pair of conductors 211 and213 in electronics housing 71 are connected to conductors 205 and 207.Conductors 211 and 213 are connected to opposite ends of a center tappedprimary winding of a transformer 215 within electronics housing 71. Thecenter tap of this primary winding within electronics housing 71operates at the same DC potential as the DC potential impressed on thecenter tap of the primary winding of transformer 201 within electronicshousing 73.

A conductor 217 is connected between the center tap of the primarywinding of transformer 215 to one end of a resistor 219 that forms partof an on/off control circuit arrangement generally indicated at 221.

Circuit arrangement 221 produces a transmitter on/off control signalthat is high to define a command to turn transmitter 1 on, and is low todefine a command to turn transmitter 1 off. If the potential onconductor 217 is greater than +1 volt and less than +2 volts, then thetransmitter on/off control signal is high; otherwise, it is low.

The circuitry within circuit arrangement 221 includes a resistor dividernetwork comprising resistors 223, 225 and 227, and a pair ofcomparators, 229 and 231. Comparators 229 and 231 have open collectoroutputs that are wire-ANDed together.

The resistance values of resistors 223, 225, and 227 are selected sothat the potential at the inverting input of comparator 231 is +1 volt,and the potential at the non-inverting input of comparator 229 is +2volts. A capacitor 223 is provided to cooperate with resistor 219 indefining a low pass noise-rejecting filter.

As to circuitry for controlling transmitter 2, there is provided acircuit arrangement generally indicated at 235 that is located withinelectronics housing 75 and that has essentially the same configurationcircuit arrangement 221. The resistor divider network for circuitarrangement 235 applies a potential of +2 volts to the inverting inputof a comparator 237, and a potential of +4 volts to the non-invertinginput of a comparator 239.

Reference is now made to FIG. 8. The overall function of the circuitryshown in block diagram form in FIG. 8 is to provide an oscillatingtransmitter-drive signal that is coupled to a transmitting antenna coil.The oscillating transmitter-drive signal is a generally square-wavemodulated sine wave, and is produced by a closed loop arrangement suchthat the frequency of the sine wave is an exact multiple of the FRsignal; the center frequency of the sine wave is 2 Mhz. A buffer circuit241 responds to the FR signal to provide a square-wave input signal to acircuit that defines a phase comparator 243. A suitable integratedcircuit for implementing the function of phase comparator 243 ismanufactured and sold by Motorola and other companies under thedesignation MC 14568.

This phase comparator circuit produces an analog phase error signal thatis applied to a conventional voltage controlled oscillator (VCO) circuit245 that includes an 8 Mhz crystal that establishes the center frequencyof oscillation of VCO 245. A conventional Schmitt trigger circuit 247responds to the output of VCO 245 to apply a square-wave signal to adivide-by-four circuit 249. The output signal of circuit 249 is a squarewave having a 2 Mhz center frequency; it is fed back to phase comparatorcircuit 249 to define a phase lock loop, and it is also applied to oneinput of a NAND gate 251. The other input of NAND gate 251 responds to arespective one of the transmitter on/off control signals described abovewith reference to FIG. 7. Conventional power driver circuitry 253responds to the output of NAND gate 251 to produce the generallysquare-wave modulated 2 Mhz sine-wave signal that is coupled to atransmitting antenna coil.

Reference is now made to FIG. 9. The overall function of the circuitryshown in block diagram form in FIG. 9 is to produce a phase-lockedsine-wave signal with a center frequency of 1.995 Mhz, that is used as alocal oscillator signal by receiving circuitry located in electronicshousing 73. The FR signal is applied to a phase comparator circuit 255that suitably is implemented by the same integrated circuit used toimplement phase comparator 243 as described above with reference to FIG.8. The output of phase comparator circuit 255 is applied to aconventional voltage controlled oscillator (VCO) circuit 257 thatincludes a 7.982 Mhz crystal that establishes the center frequency ofoscillation of VCO 257. The output of VCO 257 is applied to aconventional Schmitt trigger circuit 259 that drives a divide-by-fourcounter 261. The output of divide-by-four counter 261 is a square wavehaving a center frequency of 1.995 Mhz. This signal is applied to acounter circuit 263 which forms part of a feedback path of a phase lockloop generally indicated at 265. Suitably, counter 263 is implemented byan integrated circuit manufactured and sold by Motorola and othercompanies under the designation MC 14569. A buffer circuit 267 respondsto the 1.995 Mhz square wave and drives a conventional band pass filtercircuit 269 which produces a 1.995 Mhz sine wave that is used as a localoscillator signal.

Reference is now made to FIG. 10. The overall function of the circuitrydepicted in block diagram and schematic form in FIG. 10 is to respond tothe local oscillator signal and one of the two receiver coil outputsignals to produce a receiver phase output signal and a receiveramplitude output signal. A conventional pre-amp circuit generallyindicated at 271 responds to the receiver pick-up signal and its outputis applied to a mixer circuit arrangement generally indicated at 273.Mixer circuit arrangement 273 includes an integrated circuit 275 thatsuitably is implemented by an integrated circuit manufactured and soldby Motorola and other companies under the designation MC 1596.

Because the frequency of the pick-up signal and the local oscillatorsignals are phase-locked to a common frequency reference and differ by 5Khz, the intermediate frequency (IF) produced by mixer circuitarrangement 273 is at 5 Khz. A band pass tuning circuit arrangementgenerally indicated at 277 passes the 5 Khz IF signal to an amplifiercircuit arrangement generally indicated at 279. An active band passfilter circuit arrangement generally indicated at 281 provides furtherbandpass filtering and provides a signal to an analog divider circuitarrangement generally indicated at 283 that includes an integratedcircuit analog computational unit 284. A suitable integrated circuitanalog computational unit is sold by Analog Devices under thedesignation AD538. Divider 283 is part of an AGC loop arrangement whichincludes an active low pass filter generally indicated at 285 andRMS-To-DC converter arrangement generally indicated at 287 and anintegrating active filter generally indicated at 289 which produces afeedback signal to multiplier circuit arrangement 283.

With reference to FIG. 11, there will now be described circuitryperforming the overall function of responding to the first and secondreceiver IF signals to produce two phase-representing analog signalshaving different scale factors.

An amplifier circuit generally indicated at 291 and a Schmitt triggercircuit generally indicated at 293 are connected in tandem to convertthe receiver 1 IF signal into a square wave that is in phase with thereceiver 1 IF signal. An amplifier circuit generally indicated at 295and a Schmitt trigger generally indicated at 297 are connected in tandemto convert the receiver 2 IF signal into a square wave signal that is180 degrees out of phase with respect to the receiver 2 IF signal.

A pair of D-type flip flops 301 and 303 are interconnected in aconventional manner to define a phase detector that produces a pulsewidth modulated (PWM) signal. An integrating circuit generally indicatedat 305 cooperates with a filter choke 307 to convert a PWM signal to ananalog phase-representing signal designated as "PHASE" in FIG. 11. ThePHASE signal has a scale factor such that a peak voltage of +5 voltsrepresents a 100° phase difference.

A non-inverting amplifier generally indicated at 309 and a filter choke311 cooperate to produce another phase representing analog controlsignal designated in FIG. 11 as "X5 PHASE". The X5 PHASE signal has ascale factor such that a peak voltage of +5 volts represents a 20° phasedifference.

The positive and negative supply voltages of +4.5 volts and -4.5 voltsfor flip flops 301 and 303 are derived from the +12 volt and -12 voltsupplies. In this regard, an integrated circuit voltage reference 313establishes a reference potential that is stable with temperature.Suitably integrated circuit 313 is an LM135 sold by NationalSemiconductor and others. The temperature-stable voltage reference isapplied to a non-inverting amplifier generally indicated at 315 whichproduces the +4.5 volt supply voltage. An inverting amplifier generallyindicated at 319 produces the -4.5 volt supply voltage.

Flip flop 301 is triggered into its set state on each positive edge inthe square wave signal produced by Schmitt trigger 293. While flip flop301 is in its set state, the PWM signal is at a potential of about +4.5volts. Flip flop 303 is triggered into its set state when each positiveedge in the square wave signal is produced by Schmitt trigger 297. The Qoutput of flip flop 303 is connected to its reset (R) input and to thereset (R) input of flip flop 301. Thus, upon being triggered into itsset state, flip flop 303 immediately resets itself and also resets flipflop 301. While flip flop 301 is in its reset state, the PWM signal isat a potential of about -4.5 volts.

In circumstances in which the receiver 1 IF signal and the receiver 2 IFsignal are exactly in phase with each other, the PWM signal is a squarewave with an amplitude of ±4.5 volts. Thus, in such circumstances, thephase signal is produced by integrating the square wave to 0 volts.

In circumstances in which the receiver 1 IF signal leads the receiver 2IF signal, the PWM signal has a wave form involving a positive pulse anda negative pulse, with the negative pulse being wider than the positivepulse. In circumstances in which the receiver 1 IF signal lags thereceiver 2 IF signal, the PWM signal has a wave form involving apositive pulse which is wider than the negative pulse. The integratedsignal in either case has a magnitude that is proportional to thedifference in phase between the two IF signals, and a sign thatindicates which one leads the other.

As indicated in FIG. 6, data processing circuitry 173 is responsive tothe phase signals produced by receiving circuitry 169. Data processingcircuitry 173 includes conventional memory means for storing sampleddata provided in the sequential operation involved in alternatelytransmitting from the first transmitting circuit means and the secondtransmitting antenna means. As explained at the outset of this detaileddescription, the difference between the phases that are alternatelymeasured constitutes data from which formation resistivity can beinferred without errors attributable to certain system tolerances orvariations.

Suitably, data processing circuitry 173 computes values of formationresistivity and communicates the computed values to the surface via mudpulse telemetry techniques.

As to determining the functional relationship between formationresistivity and the integrated signal, suitable techniques involvecollecting experimental data and applying curve-fitting techniques. FIG.12 shows experimental data collected for a sub having an 8" diameter, aX1 to R1 spacing of 30", a X2 to R2 spacing of 30", and a R1 to R2spacing of 6". The data were collected using a 10' diameter test tankand a 15" diameter air borehole.

For the data plotted in FIG. 12, a satisfactory curve-fit has beenobtained with the use of a second-order (quadratic) polynomial fitted toa log-log representation of the data. This may be written as:

    x=log(phi)

    y=ax.sup.2 +bx+c

    R=exp(y)

where phi is the phase angle in degrees, R is resistivity in ohm-meters,and a, b, and c are constants. The following table shows values for theplotted points and resistivity values obtained from these equationsusing a=-0.111651, b=-1.27811, and c=5.22370:

    ______________________________________                                        Rplotted         Phi     Rcalc                                                ______________________________________                                        20.3             4.619   20.22                                                10.35            6.816   10.58                                                6.5              9.111   6.39                                                 3.06             13.886  2.97                                                 1.35             20.439  1.42                                                 0.76             28.291  0.74                                                 ______________________________________                                    

The above-described specific embodiment of this invention is presentlypreferred, and is subject to numerous modifications within the scope ofthis invention, as defined in the following claims.

We claim:
 1. A logging apparatus for use in a wellbore for measuring anattribute of at least one of (a) said wellbore, and (b) a surroundingformation during drilling operation, as a drillstring is advanced insaid wellbore, comprising:a housing; means for coupling said housing ina selected location within said drillstring; a plurality of transmitterscarried by said housing, each transmitter being individually andselectively operable for producing an electromagnetic interrogatingsignal at a selected interrogation frequency for passage through saidwellbore and said surrounding formation; a plurality of receivers,carried by said housing in a selected position relative to saidplurality of transmitters, each receiver being tuned to receiveelectromagnetic signals at said interrogation frequency to the exclusionof other frequencies; wherein a portion of said at least one of (a) saidwellbore, and (b) said surrounding formation has an attribute valuewhich can be derived from at least one of (a) amplitude attenuation ofsaid electromagnetic interrogating signal as determined frommeasurements of said electromagnetic interrogating signal, and (b) phaseshift of said electromagnetic interrogating signal as determined frommeasurements of said electromagnetic interrogating signal; a controllermember for (a) selectively energizing said plurality of transmitterscausing said electromagnetic interrogating signal to pass through saidwellbore and said surrounding formation in a particular directionrelative to said at least one receiver, (b) selectively obtainingmeasurements of said electromagnetic interrogating signal from said atleast one receiver and (c) determining an accurate measure of saidattribute value by substantially simultaneously processing recordedsignals.
 2. A logging apparatus for use in a wellbore, according toclaim 1:wherein said plurality of receivers are located at a medialportion of said housing.
 3. A logging apparatus for use in a wellbore,according to claim 1:wherein said plurality of receivers aresubstantially symmetrically positioned relative to said plurality oftransmitters.
 4. A logging apparatus for use in a wellbore, according toclaim 1:wherein said controller member determines an accurate measure ofsaid attribute value by performing calculations utilizing measurementsof at least one of (a) amplitude attenuation of said electromagneticinterrogating signal as determined from measurements of saidelectromagnetic interrogating signal taken through at least one of saidplurality of receivers, and (b) phase shift of said electromagneticsignal as determined from measurements of said electromagneticinterrogating signal taken through at least one of said plurality ofreceivers.
 5. A logging apparatus for use in a wellbore, according toclaim 1, wherein said controller member comprises:a controller memberfor (a) selectively energizing said plurality of transmitters causingsaid electromagnetic interrogating signal to pass through said wellboreand said surrounding formation in a particular direction relative tosaid plurality of receivers, (b) selectively obtaining measurements ofsaid electromagnetic interrogating signal from at least one of saidplurality of receivers, (c) determining an accurate measure of saidattribute value, and (d) determining an accurate measure of saidattribute at a plurality of differing depths of radial investigation. 6.A logging apparatus according to claim 1:wherein said attributecomprises formation resistivity; and wherein said controller memberdetermines an accurate measure of said formation resistivity.
 7. Anapparatus according to claim 1, wherein said plurality of receiverscomprises:first and second receivers, carried by said housing in aselected position relative to said plurality of relatively closelyspaced transmitters, each receiver being tuned to receiveelectromagnetic signals at said interrogation frequency to the exclusionof other frequencies.
 8. A logging apparatus according to claim 1,wherein:said plurality of transmitters comprise first and secondtransmitters, spaced apart a selected distance on said housing, eachtransmitter being individually and selectively operable for producing anelectromagnetic interrogating signal at a selected interrogationfrequency for passage through portions of said wellbore and surroundingformation; and wherein said plurality of receivers comprise first andsecond receivers, spaced apart a selected distance on said housing in aselected position relative to said first and second transmitters, eachreceiver being tuned to receive electromagnetic signals at saidinterrogation frequency to the exclusion of other frequencies.
 9. Alogging apparatus according to claim 1, further including:at least onedrive circuit, electrically coupled between said plurality oftransmitters and said controller member, for selectively energizing saidplurality of transmitters in response to commands from said controllermember.
 10. A logging apparatus according to claim 1, wherein saidcontroller member determines an accurate measure of said attribute valueby combining samples.
 11. A logging apparatus for use in interrogating aborehole and surrounding formation, comprising:a housing; a plurality oftransmitter antennas carried by said housing; a plurality of receiverantennas carried by said housing; at least one oscillator circuit, whichis electrically coupled to said plurality of transmitter antennas forselectively energizing particular ones of said plurality of relativelyclosely spaced transmitter antennas; at least one controller which iselectrically coupled to said at least one oscillator circuit; areception circuit for utilizing said plurality of receiver antennas inmeasuring at least one attribute of at least one of (a) said boreholeand (b) said surrounding formation and providing digital data to said atleast one controller; said at least one controller being operable in aplurality of modes of operation, including:(1) a transmission mode ofoperation, wherein said at least one controller provides commands inresponse to program instructions, to produce an analog signal from saidanalog output of said at least one oscillator having a particularfrequency for energizing said plurality of transmitter antennas toinitiate propagation of an interrogating electromagnetic field, havingan interrogation frequency, through said borehole and surroundingformation; and (2) a reception mode of operation, wherein said at leastone controller samples measurements of at least one attribute of saidinterrogating electromagnetic field, made utilizing at least aparticular one of said plurality of receiver antennas and recording saidmeasurements.
 12. A logging apparatus according to claim 11:wherein saidcontroller member is operable in the following additional mode ofoperation:(3) an analysis mode of operation wherein said at least onecontroller calculates a value for an attribute of at least one of saidborehole and said surrounding formation utilizing said sampledmeasurements by combining a plurality of said sampled measurements. 13.A logging apparatus according to claim 11:wherein, during saidtransmission mode of operation, said at least one controller selectsparticular ones of said plurality of transmitter antennas and saidplurality of receiver antennas for inclusion in said transmission modeof operation, thereby selecting a particular transmission path from aplurality of available transmission paths.
 14. A logging apparatus foruse in a wellbore for measuring an attribute of at least one of (a) saidwellbore, and (b) a surrounding formation, when suspended in saidwellbore, comprising:a housing; a plurality of transmitters carried bysaid housing, each transmitter being individually and selectivelyoperable for producing an electromagnetic interrogating signal at aselected interrogation frequency for passage through said wellbore andsaid surrounding formation; at least one receiver, carried by saidhousing in a selected position relative to said plurality oftransmitters, each receiver being tuned to receive electromagneticsignals at said interrogation frequency to the exclusion of otherfrequencies; wherein a portion of said at least one of (a) saidwellbore, and (b) said surrounding formation has an attribute valuewhich can be derived from at least one of (a) amplitude attenuation ofsaid electromagnetic interrogating signal as determined frommeasurements of said electromagnetic interrogating signal taken throughsaid at least one receiver, and (b) phase shift of said electromagneticinterrogating signal as determined from measurements of saidelectromagnetic interrogating signal taken through said at least onereceiver; means for sampling signals from said at least one receiver; acontroller member for (a) selectively energizing selected pair of saidplurality of transmitters causing said electromagnetic interrogatingsignals to pass through said wellbore and said surrounding formation tosaid at least one receiver, (b) or selectively obtaining measurements ofsaid electromagnetic interrogating signals from said at least onereceiver with said means for sampling, (c) selectively obtainingmeasurements of said electromagnetic interrogating signals from said atleast one receiver with said means for sampling; and (d) mathematicallycombining a plurality of measurements in order to obtain measure of saidattribute value.
 15. A logging apparatus according to claim 14:whereinsaid attribute comprises formation resistivity; and wherein saidcontroller member determines an accurate measure of said formationresistivity.
 16. An apparatus according to claim 14, wherein said atleast one receiver comprises:first and second receivers, carried by saidhousing in a selected position relative to said plurality oftransmitters, each receiver being tuned to receive electromagneticsignals at said interrogation frequency to the exclusion of otherfrequencies.
 17. A logging apparatus according to claim 14, wherein:saidplurality of transmitters comprise first and second transmitters, spacedapart a selected distance on said housing, each transmitter beingindividually and selectively operable for producing an electromagneticinterrogating signal at a selected interrogation frequency for passagethrough portions of said wellbore and surrounding formation; and whereinsaid at least one receiver include first and second receivers, spacedapart a selected distance on said housing in a selected positionrelative to said first and second transmitters and substantiallysymmetrically arranged about said first and second transmitters, eachreceiver being tuned to receive electromagnetic signals at saidinterrogation frequency to the exclusion of other frequencies.
 18. Alogging apparatus according to claim 14, further including:at least onedrive circuit, electrically coupled between said plurality oftransmitters and said controller member, for selectively energizing saidplurality of transmitters in response to commands from said controllermember.
 19. A logging apparatus according to claim 14, wherein saidcontroller member determines an accurate measure of said attribute valueby combining samples.
 20. A method of measuring in a wellbore anattribute of at least one of (a) said wellbore, and (b) a surroundingformation, comprising the method steps of:providing a housing; locatinga plurality of transmitters on said housing, each transmitter beingindividually and selectively operable for producing an electromagneticinterrogating signal at a selected interrogation frequency for passagethrough said wellbore and said surrounding formation; locating at leastone receiver on said housing in a selected position relative to saidplurality of transmitters, each receiver being tuned to receiveelectromagnetic signals at said interrogation frequency to the exclusionof other frequencies; securing said housing in a selected locationwithin a drillstring; providing a controller member; utilizing saiddrillstring to advance said wellbore by disintegrating said formationduring drilling operations; at selected intervals during drillingoperations, utilizing said controller member for selectively energizingsaid plurality of relatively closely spaced transmitters causing saidelectromagnetic interrogating signal to pass through said wellbore andsaid surrounding formation in a particular direction relative to said atleast one receiver; selectively obtaining measurements of saidelectromagnetic interrogating signal from said at least one receiver andrecording in memory a plurality of samples; and determining an accuratemeasure of said attribute value by combining a plurality of saidplurality of samples.
 21. A method of measuring in a wellbore, accordingto claim 6:substantially symmetrically positioning said plurality ofreceivers relative to said plurality of transmitters.